Replacing a boiler fan: which part number, what it costs and what to rule out first

Short answer: the fan is, after the heat exchanger, the most expensive part of a boiler, usually €410 to €620. You can recognise a worn fan by a rattling or humming noise at start-up and by fault codes about the fan speed. First rule out dirt and a blocked flue, because they give exactly the same message. Replacement is work for a certified gas engineer, because the combustion then has to be measured and adjusted again.

What does the fan actually do?

In a modern condensing boiler the fan determines how much gas and air the burner gets. It draws in fresh air, mixes it with gas in the venturi and pushes the mixture to the burner; at the same time it forces the flue gases outside. The control unit regulates the speed and continuously checks whether the fan is running at the requested speed. If it deviates, the safety system intervenes and stops the appliance. That is exactly why a fan fault shuts the boiler down completely: without a reliable airflow, combustion is unsafe.

Signs that the fan is worn out

The first sign is almost always noise. A fan with worn bearings rattles or squeaks at start-up and gets quieter once it is up to speed. An imbalance caused by dirt on the impeller, on the other hand, gives a humming vibration that you can feel throughout the boiler. Later the fault follows: the boiler tries to start, you hear the fan spin up and then everything cuts out.

Also typical: after a reset the boiler still works a few more times and then shuts down faster and faster. That is because the fan just about reaches speed when cold, but no longer when warm.

Fault codes per brand

Almost every brand has its own code for an incorrect fan speed. The most common:

Brand	Code	Message
Intergas	8	Fan speed incorrect or fan does not start
Vaillant	F.04	Fan fault
Remeha	28	Fan not running, stuck or not receiving power
AWB	02	Fan error
Bosch	C1	Fan speed too low

If your brand is not listed, look up the code in the fault code database.

Rule this out first, before you order a fan

A fan speed message does not automatically mean the fan is broken. In practice there are three causes that give the same picture and are much cheaper.

The first is dirt. Dust and grease on the impeller make the fan heavier and slower; cleaning solves that. The second is a blocked flue or air supply: a bird's nest, leaves or ice at the roof terminal make the fan work harder than the control unit expects. If in doubt, check that the terminal on the roof is clear. The third is electrical: a loose plug, a damaged wiring harness or a PCB that no longer provides the control signal. Vaillant even has a separate code for this, F.25 for a break in the wiring harness to the burner, fan and gas valve.

What does a fan cost?

Part prices including VAT from our catalogue, excluding labour. The current price is on the product page.

Brand	Part	Price
ATAG	Fan assembly 230 V AC, manufacturer number S4310520	€ 415.45
Nefit	Fan motor top, 8738206632	€ 466.95
Vaillant	Fan, 0020266326	€ 597.95
AWB	Fan bottom, 0020093806	€ 618.95

For a boiler of twelve years or older, this is the moment to do the maths: fan plus labour versus a new appliance with warranty. For a boiler of five to eight years old, replacement is almost always the sensible choice. All fans are listed together under fans in the catalogue.

Finding the right part number

Fans differ per series and per output. A fan for a 24 kW appliance does not fit the 30 kW version of the same series, even though it looks identical: the speed range and the venturi are different. So always work like this: note the brand, type and output from the rating plate, then read the manufacturer number on the sticker of the old fan itself. That number is decisive. Use it to search in the spare parts catalogue; each part shows which appliances it fits.

If you are unsure between two variants, send us a photo of the rating plate and of the old fan. We will check it before you order.

Why this is not a DIY job

The fan sits in the sealed combustion section of the boiler, and only a certified gas engineer should work on it. Even more important than that rule is what has to happen after fitting: with a new fan the gas-air ratio changes, and it has to be measured and adjusted with a flue gas analyser. A boiler that then burns too rich or too lean uses more gas, fouls up faster and can produce carbon monoxide. You cannot measure that by ear.

So feel free to order the part yourself, but have it fitted and adjusted by an engineer who can measure the combustion. Background on safe heating can be found at Milieu Centraal.

Frequently asked questions

Can I clean a fan instead of replacing it?

If the impeller is dirty and the bearings still run quietly, cleaning often helps. If it squeaks or rattles, the bearings are worn and cleaning only postpones the problem. An engineer can usually hear the difference straight away.

Why is the fan so expensive?

It is not a simple motor but a controlled unit that is matched to each appliance, including the venturi and speed control. Manufacturers therefore supply it as a complete part; separate bearings or impellers are rarely available.

My boiler makes a noise but shows no fault. Is that serious?

Not immediately, but it is the moment to do something about it. A fan that starts to rattle usually does not last much longer. Breaking down in the heating season is more annoying than having it replaced during the autumn service.

How long does a fan last?

Ten to fifteen years with normal use. Boilers that make lots of short starts, for example because the output is too high for the house, wear out faster. Annual servicing in which the impeller is cleaned noticeably extends its life.